Astro 202Prof. Jim Bell

(jfb8@cornell.edu)Spring 2008

Lecture 21: Introduction to Astrobiology:What is Life?

But first...Paper 5 (Poster proposal) handed back Thurs.

Paper 6 is due at start of class next Tuesday

Reading Assignment (G) for this Thursday is posted online: Jakosky (1998) Sci. Am. article

Keep up with the Reading in Pale Blue Dot

You should be reading the last few chapters this week

You will need to have read this book for Paper 7! (which

will be posted online on Thursday)

A few more words about your final Posters...

Building a “Poster Talk”

A Background Poster Board(typically 40” x 32”)

We will provide you with one.Choose your color by Thursday(indicate on attendance form)

Building a “Poster Talk”Your NameAstro 202; Spring ‘08

SummaryWrite a high-level

summary or “abstract” of the results of your

study

IntroductionDescribe the “problem” or

“issue” and why it interests you; Describe the

topic’s background and history

Tables or Figures

You can use tables or graphs or photos to show data, or describe measurements or

experiments, or to list interesting facts & figures...

maybe other

tables or lists or graphs...

A summary of the main results or

highlights is often useful; Could use a bulleted list

Results

DiscussionWhat do you think?How do you interpret

the data/issue?

maybe more tables or lists or graphs in

support of your ideas...

Succinct summary of your bottom line(s)

Conclusions

References, Further reading, acknowledgments

A Catchy Title in a Nice Big Font

Building a “Poster Talk”Your NameAstro 202; Spring ‘08

SummaryWrite a high-level

summary or “abstract” of the results of your

study

IntroductionDescribe the “problem” or

“issue” and why it interests you; Describe the

topic’s background and history

Tables or Figures

You can use tables or graphs or photos to show data, or describe measurements or

experiments, or to list interesting facts & figures...

maybe other

tables or lists or graphs...

A summary of the main results or

highlights is often useful; Could use a bulleted list

Results

DiscussionWhat do you think?How do you interpret

the data/issue?

maybe more tables or lists or graphs in

support of your ideas...

Succinct summary of your bottom line(s)

Conclusions

References, Further reading, acknowledgments

A Catchy Title in a Nice Big Font

Example

Design

Only

Feel fr

ee to e

xplore d

ifferen

t ways

to inclu

de the

se cont

ent ele

ments

6

Lecture #21: Astrobiology

!Introduction to Astrobiology

– What is Life?

– Case Study: Origin of Life on Earth

• Data

• Models

• Experiments

!Reading: NSS 27, PBD 21

Astrobiology

!Astrobiology (sometimes called Exobiology)

is the study of life in the Universe

– Origin of Life

– Distribution of Life

– Ultimate Fate of Life

!Astrobiology today is an extrapolation from

the one known example (Earth life), using

the basic principles of chemistry & physics

and observing the Universe around us

What is Life?

!The question is philosophical, poetic,

spiritual, intangible...but also scientific

!To find life we must define what we seek

!"Intuitive" definition:

– “I know it when I see it”

• Not very rigorous

• Applies only to Earth life?

A Quote from

Carl Sagan

What is Life?

!A more rigorous definition:– Something is alive if it has the ability to ingest

nutrients, give off waste products, & reproduce

– But what are nutrients?

– What are waste products?

– Is growth important? (mountains "grow"...)

!Definition must acknowledge that life is hard to define and that there are likely to be exceptions to any rules proposed

Attributes of Living Systems

!Rather than defining life, can we describe it

in terms of specific attributes?

!Life has at least two unique attributes:

(1) A living system must be able to reproduce, to

mutate, and to reproduce its mutations

(2) A living system must be able to convert

external energy sources into useable internal

energy sources

But even this gets dicey...

!There are systems with one attribute but not both

– Chemical Reactions

• CO2 "reproduces"

• But it's not alive!

– Crystals

• "reproduce" in regular patterns, get distorted (mutated)

– Fire

• Uses "nutrients", converts energy, "grows", "reproduces", ...

– Many other "fuzzy" cases...

• Mules? A virus? Computer programs? Robots?

CO2 + sunlight --> CO + O

H2O + sunlight --> H + OH

CO2 + OH --> CO2 + H

The Essential Requirements

! Liquid Water

– "Medium" for the chemistry of life (mobility, nutrients)

– Stable over wide range of temperatures

– Unique freezing properties help maintain stability

– Complex organic compounds don't dissolve in water!

! Source of Excess Energy

– Sunlight (photosynthesis)

– Chemical (oxidation)

– Thermal or geothermal...

! Source of Organic Molecules

– C, H, N, O, P, S combined in both simple and complex ways

– "Simple" organic molecules appear to be abundant out there…

Group Discussion

Break up into groups of 3-5 with one person acting as the note taker, and try to reach consensus on an

answer to the following question:

Do you think that there is life beyond Earth?

Explain why or why not...

5 minutes

Then I will poll the note-takers...

Origin of Life on Earth

!Key Questions:

(1) Did life originate on Earth or in space?

(2) If life originated on Earth:

(a) What were the conditions like on early Earth that

made possible the origin of life?

(b) Did life originate on or near the surface, below the

surface, or in the oceans?

"Panspermia"

! Swedish chemist Svante Arrhenius proposed in 1908 that

life is ubiquitous in the Cosmos and that "spores" or the

seeds of life were delivered to Earth essentially by accident

– No attempt to explain how life originated, only how it got to Earth

– How did the "spores" get off other planets? (impacts?)

– How did the "spores" survive harsh interstellar radiation?

! More recent variation: "intentional" panspermia

– Life was planted on Earth by space travelers

– Popular among science fiction fans and conspiracy groupies

– Still doesn't explain how life originated though...

Organic molecules in meteorites

!Some complex organic molecules (molecules containing carbon) have been found in some of the most primitive carbonaceous chondrite meteorites– Alkanes, benzene, paraffins, amino acids, …

e.g., Glycine: C2NH5O2e.g., Benzene: C6H6

Organic molecules in exotic places

!Complex organic molecules have also been found

or inferred to exist in:

–Interstellar molecular clouds

–Comets

–Interplanetary dust particles

–Some dark asteroids, rings, & planetary satellites

–Some other “anomalous” meteorites (e.g., ALH84001)

!Did life on Earth originate from raw materials

brought in by the early "rain" of debris from

asteroid, comet, and cosmic dust impacts?

Could life have originated on Earth itself?

!Recall the hypothesized environment of the early

Earth (Lecture 15):

– Heating of interior, release of volatiles

• H2, H2O, CH4, and NH3

– H2O forms liquid ocean at Earth's P,T

• NH3 dissolves in water

– Result is a highly-reducing atmosphere

• H2, CH4 abundant

• Little if any free O2

!Can simulate this environment in the lab...

The Miller-Urey Experiments(early 1950s, U. Chicago)

• Water = primitive ocean

• CH4, NH3, H2 = primitive

atmosphere

• electrical discharge =

lightning

• cycling through ocean...

• RESULTS:

! - complex organic molecules

! - simple amino acids!

! - life's building blocks!

Making Primordial Soup

Life on Earth

! The Miller-Urey experiments were perhaps too simplistic, but they demonstrated that the interactions of liquid water, natural energy sources, and organic molecules leads to the production of complex organic molecules

! Even if the Urey/Miller process was not efficient enough to produce large quantities of organics, remember that organics formed elsewhere were still being delivered to the early Earth by impacts...

! The building blocks of life are abundant in the Cosmos!

! But how did the building blocks become alive ???

22

Life on Earth

! Very soon after the early Earth cooled and the impact rate slowed, life appeared

! How? No one knows...

! Miller-Urey and more than 50 years of subsequent experiments have not been able to reproduce the result

! Life has slowly increased the amount of free O2 in Earth's

atmosphere over time

! Atmosphere is in disequilibrium

Jakosky (1998)

Starting Simple...

!Life on Earth started simple

!Most life on Earth remains simple

!All life on Earth is similar at a basic level

"Tree of Life" for

Earth, based on

similarity in RNA

sequences among

all life forms (past

and present) on

this planet

(Woese, 1987) 24

Complexity is rare...

!A census of life on Earth today or 3 billion years

ago would reach the same conclusion: life on

Earth is dominated by simple bacteria!

Gould (1994)

(early Earth)

And accidents happen...

!Evolution towards more complex life

forms is not necessarily inevitable

– bacteria are very efficient life forms...

!External, even random forces play a role

– e.g., Gould's theory of "punctuated equilibrium"

!Starting over again with the same initial

conditions, could the experiment be repeated?

– Would life form at all? (hmm...)

– Would evolution follow the same path? (probably not)

Some Big Questions

!Has this happened elsewhere?– in the solar system?

– in the Galaxy?

– in the Universe?

!Can we use our knowledge of the formation and evolution of life on Earth to make predictions about the nature, distribution, and abundance of life "out there" ?

!Should we seek simple life, or complex life?

Summary

! Life is a difficult thing to define

! But all life on Earth shares similar attributes:(1) A living system must be able to reproduce, to mutate, and to reproduce its

mutations

(2) A living system must be able to convert external energy sources into useable

internal energy sources

! The raw materials for life are common in the Cosmos

– Earth may have been "seeded" by early impacts

– Or a "primordial soup" may have been brewed early on...

! However it formed, life appeared rapidly on Earth

! Simple life forms dominate life here; complexity is rare...

! Is the same true “out there?” We should try to find out...

Next Time...Lecture 22: Life in Extreme Environments...

Readings: Paper (G) posted on web site

PBD 22